It's the way the adapters are wired. Most of them tie the ground and the negative together. The negative should be tied together with the positive on the RCA end. You can also try lifting the ground on the amp end. It has a ground lift switch on the back I believe.
Crown Xti-2000 Hum
Just hooked up my Crown Xti-2000 to a Crown IC-150 pre-amp, and I have a lot of hum and some buzz. The IC-150 pre-amp outputs are going into a Bose 901 eq then into the Xti-2000 with an RCA to XLR cable. The hum is ridiculous. how do I solve this? I had and Onkyo M504 hooked up to the system just before I hooked up the Crown and there was no hum at all. Can I run the amp with nothing in the inputs just to see if it's the unbalanced leads going into a balanced inputs causing the problem?